The word Pharmacology is derived from Greek words pharmakon (=drug= remedy= to do good) and logos

THERAPEUTIC INDEX

• Therapeutic index is the ratio between the median lethal dose and the median effective dose.

So, therapeutic index =LD50/ED50

[In experimental animal]

Or, Therapeutic index is the ratio between median toxic dose and the median effective dose.

So, therapeutic index =TD50/ED50

[In clinical medicine]

THE DOSE-RESPONSE RELATIONSHIP

Half life

Definition: The time by which the effect of a drug will come to half is called the Biological half life of that drug.

Example: 4 mg of a drug is administered. Blood concentration become 2mg (50%) after 10 minutes. So, plasma half-life =10 minutes.

Importance:

• Half life gives gross idea about the Pharmacokinetics and Pharmacodynamics of a drug.
• Half life gives idea about the duration of action of a drug.
• It can guide the dosage schedules.
• Low Half life → Frequent administration
• High half life→ Should be given once or twice daily

Bioavailability

• Definition: It is the amount or percentage of active drug that is being absorbed from a given dosage form & is made available to the site of action is called the Bioavailability of that particular from.
• e.g. Bioavailability of Paracetamol is 50%. It means if a patient orally takes 500 mg of Paracetamol, only 250mg (50%) of drug will reach to the systemic circulation.

Bioavailability

Dose
Destroyed
in gut
Not
absorbed
Destroyed
by gut wall
Destroyed
by liver
to
systemic
circulation
Plasma concentration
Time (hours)
i.v. route
oral route
Bioavailability
(AUC)o
(AUC)iv

Drug interaction

• Drug interaction is a phenomenon which occurs when the effects of one drug are modified by the prior or concurrent administration of another drug. Drug interaction may result beneficial or harmful effects. Drug interaction may occur-
• Out side the body- Phenytoin + Infusion fluid = Precipitation
• At absorption level- Antacid + Tetracycline = Decreased absorption of Tetracycline
• At distribution level- Sulfonamide + Aspirin = Sulfonamide toxicity
• At bio-transformation level- Warfarin + Barbiturates = Decreased anticoagulation
• At excretion level- Penicillin + Probenecid = Increased penicillin level
• At receptor level (Pharmacodynamics) – Thiazide diuretic + Beta Blocker = Hypertension

Route of Drug Administration:

• Oral
• Intravenous
• Intra-arterial
• Intramuscular
• Subcutaneous
• Intraperitoneal

• Topical
• Suppository
• Inhalation
• Sub-lingual
• Local injection
• Intrathecial

DISPOSITION OF DRUGS

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